Acidic mine drainage (AMD) is a pervasive environmental problem around the world. It affects, for example, over 23,000 kilometers of streams in the United States alone. AMD forms when mining activities expose sulfide minerals to the near-surface environment and oxygen-rich water. The result is the aqueous transfer of metals (e.g. Fe, Al, and Mn) and low pH water (pH less than 4) to streams, lakes, and aquifers. This often contributes to the destruction of aquatic habitat and organisms. Impact may continue indefinitely even after mining ceases. Many present-day problem sites are mines that have been abandoned for over 100 years. Long-term, cost-effective treatment techniques for mine drainage have been avidly pursued, particularly in regions such as the western United States where between 20,000 and 50,000 mines are currently generating acidity. In these situations, techniques that are passive and require little or no maintenance are most desirable. Constructed wetlands or anoxic limestone drains are two commonly used options. They trigger the precipitation of dissolved metals from the water through reduction of acidity or other mechanisms.
Pyrite (FeS2), a major source of acidic mine drainage, oxidizes according to the following reactions:FeS2(s)+ 7/2O2+H2O=Fe2++2SO42−+2H+  (1)Fe2++¼O2+H+=Fe3++½H2O  (2)Fe3++3H2O=Fe(OH)3(s)+3H+  (3)
The oxidation of one mole of pyrite releases four moles of H+, typically generating waters with pH values of 2 to 3. Under abiotic conditions the rate-determining step in the sequence of reactions is the oxidation of ferrous iron by reaction (2) which is negligible (a half-life of many years) below a pH of 5.5. With biotic mediation, however, microbial catalysis by autotrophic iron bacteria such as Thiobacillus ferrooxidans dramatically increases the rate of ferrous iron oxidation.
The vast majority of mines in the world occur in drainage basins containing no exposed limestone that could act as a natural neutralizing agent. However, on the Western Cumberland Plateau Escarpment of Tennessee (USA) where coal has been historically mined several stream basins exist where limestone is exposed.
Treatment techniques for mine drainage, such as wetlands or anoxic limestone drains, have focused on the reduction of acidity and the precipitation of dissolved metals from the water. Limestone, in both oxic and anoxic settings, is inexpensive and has been frequently used to neutralize acidity. In the presence of oxygen (oxic conditions), neutralization promotes the precipitation of iron oxide, along with other trace oxides. Limestone, by itself, is not an ideal solution. For example, the use of limestone by itself results in an undesirable side effect. Neutralization by limestone increases precipitation of amorphous iron hydroxide. Upon formation, the precipitate coats the limestone surface, ultimately inhibiting further neutralization, and causing failure of the remedial system. Anoxic limestone drains control this problem by excluding oxygen and preventing reactions (2) and (3) from proceeding, so no iron hydroxide precipitates.
The precipitate formed by hydrolyzing ferric iron (as in reaction (3) above) is usually cited as amorphous ferrihydrite and its composition is given for simplicity as Fe(OH)3. The actual precipitate is a mixture of phases including goethite, ferrihydrite, jarosite and an oxyhydroxysulfate of iron. The ferrihydrite precipitated by acidic mine drainage may contain substantial quantities of elements other than iron, particularly silica, manganese, sulphate, aluminum and arsenic. Precipitates from the oxidation of ferrous iron play an important role in the removal of trace metals (e.g. Cu, Zn, Cd, Pb, and As) from mine drainage because the of the precipitates' low crystallinity and high surface area make them very effective in adsorbing trace metals.
The overall rate of iron removal in streams is highly variable, with reported values ranging from 10−1 to 10−6 mol/L/s. Ferrihydrite precipitated by AMD may contain substantial quantities of elements other than iron, particularly silica, manganese, sulfate, aluminum and arsenic. For these reasons, it is beneficial to promote the precipitation of iron.
An additional concern associated with using limestone by itself relates to its known effect on solution chemistry. It has been shown that the high concentrations of bicarbonate in solution from limestone addition result in significant sulfate release from precipitated iron oxyhydroxides, a previously unreported negative consequence of limestone neutralization.
The remediation of streams adversely affected by acidic mine drainage often employs limestone as a neutralization agent. However, metal precipitation on the limestone surface can render it ineffective. Additionally, large amounts of sulfate may be released using this method. Leaving the water acidic harms aquatic plant and animal life and leaves the water contaminated with harmful metals. An improved method of neutralizing the acidic waters and removing the metals from the waters is therefore needed.
Acidified waters can also come from sources other than mine drainage. Acid rain is responsible for damaging aquatic life, the environment and public infrastructure especially in the northeast portion of the United States. Natural waters, such as lakes, rivers, reservoirs, creeks and streams, in this region are routinely found to have very low pH levels. Attempts to remediate the acidified waters have met with limited success for the reasons outlined above. There is still a need for a method of neutralizing acidified waters caused by acid rain that (1) is not rendered ineffective by a coating of precipitated metals, (2) does not release sulfates and (3) removes unwanted metals from the affected waters.
Industry also faces a challenge with acidified waste or process streams or waste or process streams that contain metal contaminates. City and municipal drinking, sewage, storm and waste water plants also have a need for an improved method of deacidifying or removing metals dissolved or suspended in their waste or process streams.